1. Field of the Invention
The present invention relates to a polymer dispersed liquid crystal display, and more particularly to a color polymer dispersed liquid crystal display, which reduces changes in properties caused by unstable phase separation and produces improved colors, and a method for manufacturing the same.
2. Description of the Prior Art
A polymer dispersed liquid crystal display (“PDLCD”) has a structure wherein a PDLC film consisting of liquid crystal droplets, which have a diameter of 1 to 2 μm and are dispersed in a polymer film, instead of a conventional liquid crystal film consisting only of liquid crystal molecules, is disposed between a pair of transparent substrates. The PDLCD requires no polarizer and can be easily manufactured due to its flexibility. Applications of the PDLCD with improved brightness include widescreen display devices and light-modulating window projection TVs.
If an electric field is applied to a PDLC film inserted between a pair of driving electrodes, a director of the liquid crystal will be oriented in the direction of the electric field. When the ordinary refractive index no of the liquid crystal is equal to the refractive index np of the polymer, the PDLC film becomes transparent by light penetration, thereby making the PDLCD appear white. When the electric field is removed, the director of the liquid crystal returns to its initially chaotic state due to the surface anchoring energy. At this time, the effective refractive index of the liquid crystal greatly deviates from the refractive index np of the polymer. The difference in refractive indexes causes interfacial light scattering, whereby the PDLC film becomes opaque and the PDLCD appears black.
As methods for dispersing liquid crystal droplets in a polymer, phase separation and emulsification are available. The phase separation is based on a principle that phase separation of an initially homogeneous liquid crystal-polymer (or precursor) mixture is achieved by polymerization, cooling or solvent evaporation. The emulsification is based on a principle that the liquid crystal in an initially heterogeneous liquid crystal-polymer aqueous solution is encapsulated by the polymer during evaporation of water. PDLC films are typically formed by phase separation.
However, when a PDLC film is formed by phase separation, it is difficult and practically impossible to achieve a complete phase separation, because an initially homogeneous phase becomes heterogeneous during phase separation by various methods. Accordingly, a PDLC film obtained by phase separation may show properties slightly different from its original properties, because the liquid crystal and the polymer (or precursor) contaminate each other. Particularly, the properties of the PDLC film are affected by minor changes in processing conditions.
Also, a color PDLC film can be formed by a guest-host method or holography. However, these methods enable only single color reflective displays. In the prior art, full color reflective displays have been achieved by combining PDLC films to form a multilayer structure. Such a stacking structure, however, has a problem in the link between layers and increases the driving voltage and the cost of manufacture.
Although a color PDLCD can be prepared using an existing color filter, UV light cannot penetrate into the color filter. Accordingly, the polymerization induced phase separation (PIPS) utilizing UV polymerization, which is the most generally used method to prepare a PDLCD, cannot be used to prepare a color PDLCD.
A color PDLCD can also be made by a color sequential display method. However, this method has technical drawbacks because it requires high-speed responsiveness.
Korean Patent Application No. 10-1999-0016162 discloses a PDLCD which enables a full-color display by a polymer film formed by mixing a polymeric material with a liquid crystal and dichroic dyes for producing particular colors. According to the disclosure of this reference, liquid crystal cells of a color PDLCD can be formed by the following two methods.
The first method forms liquid crystal cells by coating a polymer film, which contains a mixed solution of a liquid crystal and dyes, and forming polymer domains of red, green and blue colors.
According to the second method, a polymer film is coated and cured to have a plurality of pores linked to each another. A mixture of dyes and a liquid crystal is injected into the exposed pores of the cured polymer film to form liquid crystal cells. It is also possible to form the liquid crystal cells by ink-jetting a mixture of a liquid crystal and red, green and blue dyes onto a predetermined position of the cured polymer film.
The first method forms fine pores and pattern by curing a photoreactive material. As is generally known, however, it is difficult to achieve high resolution by this method. Further, the properties of the liquid crystal are affected during developing process.
Since the second method forms pores linked to each another, colors are likely to be mixed when a mixture of the liquid crystal and the dyes are injected or inkjetted. Further, although the fine pores are linked to each another, it is difficult to completely inject the liquid crystal into the polymer simply by inkjetting the mixture.
The prior art mentioned above uses dyes for producing colors. However, dyes cannot produce a full range of colors and are less reliable, when compared to pigments used in existing color filters. Particularly, dyes used in form of a mixture with a liquid crystal may deteriorate the properties of the liquid crystal.